Semiconductor device and method of forming the same

ABSTRACT

Provided are a semiconductor device and a method of forming the same. The semiconductor device includes a substrate, a plurality of semiconductor nanosheets, a bottom dielectric layer, and a gate stack. The substrate includes at least one fin. The plurality of semiconductor nanosheets are stacked on the at least one fin. The bottom dielectric layer is disposed between the at least one fin and the plurality of semiconductor nanosheets. The gate stack wraps the plurality of semiconductor nanosheets.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the priority benefit of U.S. provisionalapplication Ser. No. 62/925,770, filed on Oct. 25, 2019. The entirety ofthe above-mentioned patent application is hereby incorporated byreference herein and made a part of this specification.

BACKGROUND

Technological advances in Integrated Circuit (IC) materials and designhave produced generations of ICs where each generation has smaller andmore complex circuits than the previous generations. In the course of ICevolution, functional density (for example, the number of interconnecteddevices per chip area) has generally increased while geometry sizes havedecreased. This scaling down process generally provides benefits byincreasing production efficiency and lowering associated costs.

Such scaling down has also increased the complexity of processing andmanufacturing ICs and, for these advances to be realized, similardevelopments in IC processing and manufacturing are needed. For example,multi-gate devices have been introduced to replace planar transistors.On the other hands, the smaller feature size may lead to more leakagecurrent. As the demand for even smaller electronic devices has grownrecently, there has grown a need for reducing leakage current of ICs.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the followingdetailed description when read with the accompanying figures. It isnoted that, in accordance with the standard practice in the industry,various features are not drawn to scale. In fact, the dimensions of thevarious features may be arbitrarily increased or reduced for clarity ofdiscussion.

FIG. 1 to FIG. 13 are perspective views of intermediate stages in theformation of a semiconductor device in accordance with a firstembodiment of the disclosure.

FIG. 14 to FIG. 18 are corresponding fragmentary cross-sectional viewsof a semiconductor device in FIG. 7 to FIG. 8 taken along the line A-A′in accordance with some embodiments of the disclosure.

FIG. 19 to FIG. 24 are corresponding fragmentary cross-sectional viewsof a semiconductor device in FIG. 9 to FIG. 13 taken along the line A-A′in accordance with some embodiments of the disclosure.

FIG. 25 to FIG. 28 are corresponding fragmentary cross-sectional viewsof a semiconductor device in FIG. 10 to FIG. 13 taken along the lineB-B′ in accordance with some embodiments of the disclosure.

FIG. 29 to FIG. 30 are cross-sectional views of intermediate stages inthe formation of a semiconductor device in accordance with a secondembodiment of the disclosure.

DETAILED DESCRIPTION

The following disclosure provides many different embodiments, orexamples, for implementing different features of the provided subjectmatter. Specific examples of components and arrangements are describedbelow to simplify the present disclosure. These are, of course, merelyexamples and are not intended to be limiting. For example, the formationof a first feature over or on a second feature in the description thatfollows may include embodiments in which the first and second featuresare formed in direct contact, and may also include embodiments in whichadditional features may be formed between the first and second features,such that the first and second features may not be in direct contact. Inaddition, the present disclosure may repeat reference numerals and/orletters in the various examples. This repetition is for the purpose ofsimplicity and clarity and does not in itself dictate a relationshipbetween the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,”“above,” “upper” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. The spatiallyrelative terms are intended to encompass different orientations of thedevice in use or operation in addition to the orientation depicted inthe figures. The apparatus may be otherwise oriented (rotated 90 degreesor at other orientations) and the spatially relative descriptors usedherein may likewise be interpreted accordingly.

The gate all around (GAA) transistor structures may be patterned by anysuitable method. For example, the structures may be patterned using oneor more photolithography processes, including double-patterning ormulti-patterning processes. Generally, double-patterning ormulti-patterning processes combine photolithography and self-alignedprocesses, allowing patterns to be created that have, for example,pitches smaller than what is otherwise obtainable using a single, directphotolithography process. For example, in one embodiment, a sacrificiallayer is formed over a substrate and patterned using a photolithographyprocess. Spacers are formed alongside the patterned sacrificial layerusing a self-aligned process. The sacrificial layer is then removed, andthe remaining spacers may then be used to pattern the GAA structure.

The present disclosure is generally related to semiconductor devices andthe fabrication thereof, and more particularly to multi-gatetransistors. Multi-gate transistors include those transistors whose gatestructures are formed on at least two-sides of a channel region. Thesemulti-gate devices may include a p-type metal-oxide-semiconductor deviceor an n-type metal-oxide-semiconductor multi-gate device. Specificexamples may be presented and referred to herein as FINFET, on accountof their fin-like structure. Also presented herein are embodiments of atype of multi-gate transistor referred to as a gate-all-around (GAA)device. A GAA device includes any device that has its gate structure, orportion thereof, formed on 4-sides of a channel region (e.g.,surrounding a portion of a channel region). Devices presented hereinalso include embodiments that have channel regions disposed in nanosheetchannel(s), bar-shaped channel(s), and/or other suitable channelconfiguration. Presented herein are embodiments of devices that may haveone or more channel regions (e.g., nanosheets) associated with a single,contiguous gate structure. However, one of ordinary skill wouldrecognize that the teaching can apply to a single channel (e.g., singlenanosheets) or any number of channels. One of ordinary skill mayrecognize other examples of semiconductor devices that may benefit fromaspects of the present disclosure.

In accordance with some embodiments, a bottom dielectric layer is formedbelow a gate stack to block the punch through leakage current betweenadjacent source/drain (S/D) regions, thereby enhancing the deviceperformance and the reliability. In addition, the bottom dielectriclayer may prevent dopants in the substrate (e.g., phosphorous in then-well and boron in the p-well) from diffusing upwardly into the S/Dregions, thereby maintaining the device performance. Moreover, thebottom dielectric layer may combine APT region above and below thebottom dielectric layer to further block the punch through leakagecurrent.

FIG. 1 to FIG. 13 are perspective views of intermediate stages in theformation of a semiconductor device in accordance with a firstembodiment of the disclosure. The semiconductor device illustrated inthe following embodiments may be applied to, but not limited thereto, afin field-effect transistor (FinFET), gate-all-around (GAA) FET, orother transistors including a multi-gate.

Referring to FIG. 1, a substrate 100 is provided. In some embodiments,the substrate 100 includes a crystalline silicon substrate (e.g.,wafer). The substrate 100 may include various doped regions (e.g.,p-type well and/or n-type well) depending on design requirements. Insome embodiments, the doped regions may be doped with p-type or n-typedopants. For example, the doped regions may be doped with p-typedopants, such as boron or BF₂; n-type dopants, such as phosphorus orarsenic; and/or combinations thereof. The doped regions may beconfigured for an n-type FinFET, or alternatively, configured for ap-type FinFET. For clarity, the doped regions are not illustrated inFIG. 1 and subsequent drawings. In some alternative embodiments, thesubstrate 100 includes an element semiconductor such as silicon orgermanium, a compound semiconductor such as silicon carbide, galliumarsenic, gallium phosphide, indium phosphide, indium arsenide and indiumantimonide, an alloy semiconductor such as SiGe, GaAsP, AlInAs, AlGaAs,GaInAs, GaInP and GaInAsP or combinations thereof.

As shown in FIG. 1, a bottom semiconductor material 204 is then formedon the substrate 100. In some embodiments, the bottom semiconductormaterial 204 includes SiGe, Ge, Si, or a combination thereof, and may beformed by an epitaxial growth process, such as a molecular beam epitaxy(MBE) process, a metalorganic chemical vapor deposition (MOCVD) process,or the like. In the case, the bottom semiconductor material 204 may bean epitaxial SiGe layer. In some alternative embodiments, the bottomsemiconductor material 204 is formed by a suitable deposition, such aschemical vapor deposition (CVD), atomic layer deposition (ALD), or thelike. In the case, the bottom semiconductor material 204 may be apoly-SiGe layer. In some other embodiments, the bottom semiconductormaterial 204 has a thickness T3 in a range from about 5 nm to about 10nm.

Thereafter, a buffer material 206 is formed on the bottom semiconductormaterial 204, and the bottom semiconductor material 204 is sandwichedbetween the buffer material 206 and the substrate 100. In someembodiments, the buffer material 206 includes an undoped silicon layer,a SiGe layer, or a combination thereof, that has a lower resistance thanthe substrate 100. In some alternative embodiments, the buffer material206 and the bottom semiconductor material 204 have different materials.The buffer material 206 may be formed by an epitaxial growth process,such as a MBE process, a MOCVD process, or the like. In the case, thebuffer material 206 may be an epitaxial Si layer. In some alternativeembodiments, the buffer material 206 is formed by a suitable deposition,such as CVD, ALD, or the like. In the case, the buffer material 206 maybe a poly-Si layer. In some other embodiments, the buffer material 206has a thickness greater than the thickness T3 of the bottomsemiconductor material 204.

As shown in FIG. 1, a semiconductor stack 102 is formed on the buffermaterial 206. The semiconductor stack 102 may include a plurality offirst layers 104 a, 104 b, 104 c (collectively referred to as “firstlayers 104”) and a plurality of second layers 106 a, 106 b, 106 c(collectively referred to as “second layers 106”) stacked alternately ina Z direction. Although only three first layers 104 and three secondlayers 106 are illustrated in FIG. 1, the embodiments of the presentdisclosure are not limited thereto. In other embodiments, the number ofthe first layers 104 and the second layers 106 are adjusted by the need,such as one first layer, two first layers, four first layers, or morefirst layers. The number of the second layers corresponds to the numberof the first layers.

In some embodiments, the first layers 104 and the second layers 106include different materials. For example, the first layers 104 are SiGelayers having a germanium percentage in the range between about 10 wt %and 40 wt %, and the second layers 106 are Si layers free fromgermanium. However, the embodiment of the disclosure is not limitedthereto, in other embodiments, the first layers 104 and the secondlayers 106 have materials with different etching selectivities. In someembodiments, the first layers 104 and the second layers 106 are formedby an epitaxial growth process, such as a MBE process, a MOCVD process,or the like. In the case, the first layers 104 are epitaxial SiGelayers, and the second layers 106 are epitaxial Si layers. In somealternative embodiments, the first layers 104 and the second layers 106are formed by a suitable deposition, such as CVD, ALD, or the like. Inthe case, the first layers 104 are poly-SiGe layers, and the secondlayers 106 are poly-Si layers.

In some embodiments, the first layers 104 have the same thickness T1 andthe second layers 106 have the same thickness T2. In some embodiments,the thickness T1 may be in a range from about 5 nm to about 20 nm andthe thickness T2 may be in a range from about 5 nm to about 20 nm.Alternatively, the first layers 104 a, 104 b, and 104 c may havedifferent thicknesses, and the second layers 106 a, 106 b, and 106 c mayhave different thicknesses. In some other embodiments, the first layers104 and the second layers 106 have the same or different thicknesses. Insome embodiments, the thickness T3 of the bottom semiconductor material204 is less than the thickness T1 of the first layers 104 and/or lessthan the thickness T2 of the second layers 106. A ratio of the thicknessT1 or T2 to the thickness T3 may be greater than or equal to 2, namely,T1/T3≥2 or T2/T3≥2.

As shown in FIG. 1, a mask layer 108 is formed on the semiconductorstack 102. The mask layer 108 may include a single-layered structure, atwo-layered structure, or a multi-layered structure. For example, themask layer 108 includes a silicon oxide (SiO) layer and a siliconnitride (SiN) layer on the SiO layer. In some embodiments, the masklayer 108 is formed by CVD, ALD, or the like.

Referring to FIG. 1 and FIG. 2, the mask layer 108 is patterned to forma plurality of mask strips 118. The semiconductor stack 102, the buffermaterial 206, the bottom semiconductor material 204, and the substrate100 are then patterned by using the mask strips 118 as a mask, so as toform a plurality of trenches 10. In the case, a plurality of fins 111, aplurality of bottom semiconductor layers (or strips) 214, a plurality ofbuffer layers (or strips) 216, and a plurality of stacks ofsemiconductor strips 112 are formed between the trenches 10. As shown inFIG. 2, the trenches 10 extend into the substrate 100, and havelengthwise directions parallel to each other. Herein, the stacks ofsemiconductor strips 112 may be referred to as nanosheet stacks 112 andthe combination of the fins 111, the bottom semiconductor layers 214,the buffer layers 216, and the nanosheet stacks 112 may be referred toas hybrid fins 110, alternatively. Although only two hybrid fins 110 areillustrated in FIG. 2, the embodiments of the present disclosure are notlimited thereto. In other embodiments, the number of the hybrid fins 110may be adjusted by the need, such as one hybrid fin, three hybrid fins,four hybrid fins, or more hybrid fins. In addition, the mask strips 118illustrated in FIG. 2 have flat top surfaces. However, the embodimentsof the present disclosure are not limited thereto. In other embodiments,the mask strips 118 may have dome top surfaces due to the high aspectratio etching.

As shown in FIG. 2, in some embodiments, the nanosheet stack 112 includea plurality of first nanosheets 114 a, 114 b, 114 c (collectivelyreferred to as “first nanosheets 114”) and a plurality of secondnanosheets 116 a, 116 b, 116 c (collectively referred to as “secondnanosheets 116”) stacked alternately along a Z direction and extendingalong a Y direction. The bottom semiconductor layers 214 and the bufferlayers 216 are disposed between the fins 111 and the nanosheet stacks112 and also extending along the Y direction.

Referring to FIG. 2 and FIG. 3, a plurality of insulating layers 113 areformed in trenches 10. In detail, in some embodiments, an insulatingmaterial is formed on the substrate 100 to cover the hybrid fins 110 andto fill up the trenches 10. In addition to the hybrid fins 110, theinsulating material further covers the mask strips 118. The insulatingmaterial may include silicon oxide, silicon nitride, silicon oxynitride,a spin-on dielectric material, or a low-k dielectric material. Herein,the low-k dielectric materials are generally dielectric materials havinga dielectric constant lower than 3.9. The insulating material may beformed by flowable chemical vapor deposition (FCVD), high-density-plasmachemical vapor deposition (HDP-CVD), sub-atmospheric CVD (SACVD), orspin on. A planarization process may be performed, to remove a portionof the insulating material and the mask strips 118 until the hybrid fins110 are exposed. In the case, as shown in FIG. 3, top surfaces 110 t ofthe hybrid fins 110 are substantially coplanar with a top surface 113 tof the planarized insulating material or insulating layers 113. In someembodiments, the planarization process includes a chemical mechanicalpolish (CMP), an etching back process, a combination thereof, or thelike.

Referring to FIG. 3 and FIG. 4, the insulating layers 113 are recessedto form a plurality of isolation regions 115. After recessing theinsulating layers 113, the hybrid fins 110 protrude from top surfaces115 t of the isolation regions 115. That is, the top surfaces 115 t ofthe isolation regions 115 may be lower than the top surfaces 110 t ofthe hybrid fins 110. In some embodiments, the nanosheet stacks 112, thebuffer layers 216, and the bottom semiconductor layers 214 are exposedby the isolation regions 115. That is, the top surfaces 115 t of theisolation regions 115 may be substantially coplanar with or lower thanbottom surfaces 214 bt of the bottom semiconductor layers 214. Further,the top surfaces 115 t of the isolation regions 115 may have a flatsurface as illustrated, a convex surface, a concave surface (such asdishing), or a combination thereof. In some embodiments, the insulatinglayers 113 are recessed by using an appropriate etching process, such asa wet etching process with hydrofluoric acid (HF), a dry etchingprocess, or a combination thereof. In some embodiments, a heightdifference H between the top surfaces 110 t of the hybrid fins 110 andthe top surfaces 115 t of the isolation regions 115 ranges from about 30nm to about 100 nm. In some embodiments, the isolation regions 115 maybe shallow trench isolation (STI) regions, deep trench isolation (DTI)regions, or the like.

Referring to FIG. 4 and FIG. 5, a dummy dielectric layer 120 is formedon the substrate 100. In detail, as shown in FIG. 5, the dummydielectric layer 120 conformally cover the surfaces of the nanosheetstacks 112, the buffer layers 216, the bottom semiconductor layers 214,and the top surfaces 115 t of the isolation regions 115. In someembodiments, the dummy dielectric layer 120 includes silicon oxide,silicon nitride, silicon oxynitride, or the like, and may be formed byCVD, ALD or the like. In some alternative embodiments, the dummydielectric layer 120 and the isolation regions 115 have the same ordifferent dielectric materials.

Referring to FIG. 6, a dummy gate stack 122 is formed on portions of thenanosheet stacks 112, portions of the buffer layers 216, portions of thebottom semiconductor layers 214, and portion of the isolation regions115. The dummy gate stack 122 may extend along a X directionperpendicular to the extending direction of the nanosheet stacks 112,the buffer layers 216, and the bottom semiconductor layers 214. That is,the dummy gate stack 122 may be formed across the nanosheet stacks 112,the buffer layers 216, and the bottom semiconductor layers 214.

Specifically, the dummy gate stack 122 may include dummy gate electrode124 and a portion of the dummy dielectric layer 120 covered by the dummygate electrode 124. Herein, the portion of the dummy dielectric layer120 covered by the dummy gate electrode 124 may be referred to as dummygate dielectric layer 120 m. In some embodiments, the dummy gateelectrode 124 includes a silicon-containing material, such aspoly-silicon, amorphous silicon, or a combination thereof. The dummygate electrode 124 may be formed by using a suitable process, such asALD, CVD, PVD, plating, or combinations thereof. Although the dummy gateelectrode 124 illustrated in FIG. 6 is a single-layered structure, theembodiments of the present disclosure are not limited thereto. In otherembodiments, the dummy gate electrode 124 may be a multi-layeredstructure. The dummy gate stack 122 may also include hard mask layer 126over dummy gate electrode 124. In some embodiments, the hard mask layer126 includes a single-layered structure, a two-layered structure, amulti-layered structure. For example, the hard mask layer 126 includes asilicon oxide layer 126 a and a silicon nitride layer 126 b over thesilicon oxide layer 126 a.

As shown in FIG. 6, a pair of spacers 128 are also formed on sidewallsof the dummy gate stack 122. In some embodiments, the spacers 128 andthe dummy gate stack 122 have the same extending direction, namely, theX direction. Similar to the dummy gate stack 122, the spacers 128 arealso formed across the nanosheet stacks 112, the buffer layers 216, andthe bottom semiconductor layers 214. In some embodiments, the spacers128 are formed of dielectric materials, such as silicon oxide, siliconnitride, carbonized silicon nitride (SiCN), SiCON, or a combinationthereof. Although the spacers 128 illustrated in FIG. 6 is asingle-layered structure, the embodiments of the present disclosure arenot limited thereto. In other embodiments, the spacers 128 may be amulti-layered structure. For example, the spacer 128 may include asilicon oxide layer and a silicon nitride layer on the silicon oxidelayer. As shown in FIG. 6, the dummy gate stack 122 and the spacers 128cover middle portions of the nanosheet stacks 112, the buffer layers216, the bottom semiconductor layers 214, and reveal the opposite endportions not covered.

Referring to FIG. 6 and FIG. 7, the end portions of the nanosheet stacks112 are removed and recessed to form recesses 12. Herein, the recesses12 may be referred to as source/drain (S/D) recesses 12. In someembodiments, the end portions of the nanosheet stacks 112 may be removedby an anisotropic etching process, an isotropic etching process, or acombination thereof. In some embodiments, the S/D recesses 12 furtherextend into the buffer layers 216 and are not in contact with the bottomsemiconductor layers 214. That is, the end portions of the nanosheetstacks 112 are entirely removed and portions of the buffer layers 216are further removed. In the case, as shown in FIG. 7, the S/D recesses12 have bottom surfaces 12 bt lower than the top surfaces 115 t of theisolation regions 115 and a non-zero distance D (or a portion of thebuffer layer 216) is included between the bottom surfaces 12 bt of theS/D recesses 12 and top surfaces 214 t of the bottom semiconductorlayers 214. In some embodiments, the non-zero distance D ranges fromabout 5 nm to about 50 nm. In addition, some portions of the dummydielectric layer 120 are removed and other portions of the dummydielectric layer 120 may be left standing over and aligned to the edgesof isolation regions 115, with the S/D recesses 12 formed therebetween.Further, portions of the dummy dielectric layer 120 (in FIG. 6) coveredby the spacers 128 may be considered to as portions of the spacers 128as shown in FIG. 7. The spacers 128 may cover sidewalls of the dummygate stack 122 which includes the dummy gate dielectric layer 120 m, thedummy gate electrode 124, and the hard mask layer 126.

It should be noted that, after forming the S/D recesses 12, a pluralityof inner spacers 132 may be formed before forming source/drain (S/D)regions 140 (as shown in FIG. 8). For clarity, the forming steps areshown in FIG. 14 to FIG. 18 which correspond fragmentary cross-sectionalviews of a semiconductor device in FIG. 7 to FIG. 8 taken along the lineA-A′.

Referring to FIG. 14, after forming the S/D recesses 12, a sidewall 112s of the he nanosheet stacks 112 may be aligned with an outer sidewall128 s of the spacers 128. For clarity, the dummy gate stack 122 in thefollowing cross-sectional views are illustrated as a single layer.

Referring to FIG. 14 and FIG. 15, portions of the first nanosheets 114are laterally recessed. In some embodiments, the portions of the firstnanosheets 114 exposed by the S/D recesses 12 are removed, and thus asshown in FIG. 15, a plurality of cavities 14 a, 14 b, and 14 c(collectively referred to as “cavities 14”) are respectively formedbetween the second nanosheets 116. In some embodiments, the firstnanosheets 114 are laterally recessed by a wet etching, a dry etching,or a combination thereof. For example, the first nanosheets 114 may beselectively etched by using a wet etchant such as, but not limited to,ammonium hydroxide (NH₄OH), tetramethylammonium hydroxide (TMAH),ethylenediamine pyrocatechol (EDP), or potassium hydroxide (KOH)solutions. Alternatively, before laterally recessing the portions of thefirst nanosheets 114, the end portions of the first nanosheets 114exposed by the recesses 12 may be selectively oxidize, so as to increasethe etching selectivity between the first and second nanosheets 114 and116. In some alternative embodiments, the oxidation process may beperformed by exposing to a wet oxidation process, a dry oxidationprocess, or a combination thereof. The chemical used in the oxidationprocess may include H₂SO₄ or the like.

In some embodiments, as shown in FIG. 15, the cavities 14 a, 14 b, and14 c have different (lateral) depths D1, D2, and D3. In the case, theuppermost first nanosheet 114 c is in contact with the etchant for alonger time than the underlying first nanosheet 114 a, and thus aremoval amount of the uppermost first nanosheet 114 c is greater than aremoval amount of the underlying first nanosheet 114 b. Similarly, theremoval amount of the first nanosheet 114 b is greater than a removalamount of the underlying first nanosheet 114 a. Accordingly, as shown inFIG. 15, the cavity 14 c has the depth D3 greater than the depth D2 ofthe cavity 14 b, and the cavity 14 a has the depth D1 less than thedepth D2 of the cavity 14 b. That is, the depths D1, D2, and D3gradually increase from bottom to top, namely, D1<D2<D3. In someembodiments, the sidewalls 114 s of the first nanosheets 114 are notaligned with each other. The depth D1 may be in a range from about 5 nmto about 15 nm, the depth D2 may be in a range from about 6 nm to about16 nm, the depth D3 may be in a range from about 7 nm to about 17 nm.The sidewalls 114 s of the first nanosheets 114 illustrated in FIG. 15are curved or arc sidewalls protruding from the end of the firstnanosheets 114 into the center of the first nanosheets 114. However, theembodiments of the present disclosure are not limited thereto. In otherembodiments, the sidewalls of the first nanosheets 114 are verticalsidewalls substantially perpendicular to the top surface of thesubstrate 100. In some alternative embodiments, the cavities 14 a, 14 b,and 14 c have the same (lateral) depth, namely D1=D2=D3.

Referring to FIG. 15 and FIG. 16, an inner spacer material layer 130 isformed on the substrate 100. In some embodiments, the inner spacermaterial layer 130 conformally covers the S/D recesses 12, the dummygate stack 122, and the spacers 128, and further fills in the cavities14 (including cavities 14 a-14 c) to reduce the size of the cavities 14or completely fill in the cavities 14. In some embodiments, the innerspacer material layer 130 includes silicon oxides, silicon nitrides,silicon carbides, silicon carbide nitride, silicon oxide carbide,silicon carbide oxynitride, and/or other suitable dielectric materials,and may be formed by ALD or any other suitable method. In somealternative embodiments, the inner spacer material layer 130 include alow-k dielectric material having a dielectric constant lower than 3.9.

Referring to FIG. 16 and FIG. 17, a portion of the inner spacer materiallayer 130 is removed to form a plurality of inner spacers 132 a, 132 b,and 132 c (collectively referred to as “inner spacers 132”) respectivelyin the cavities 14 a, 14 b, and 14 c. In some embodiments, the portionof the inner spacer material layer 130 is removed by a plasma dryetching or any other suitable method. Generally, the plasma dry etchingetches a layer in wide and flat areas faster than a layer in concave(e.g., holes, grooves and/or slits) portions. Thus, the inner spacermaterial layer 130 may remain inside the cavities 14. The remainingportions of the inner spacer material layer 130 is referred to as theinner spacers 132. In some embodiments, as shown in FIG. 17, the innerspacers 132 may be formed as crescent shape. That is, one of the innerspacers 132 has a curved inner sidewall 132 s 1 and a curved outersidewall 132 s 2. The outer sidewalls 132 s 2 are dented or concave fromthe sidewalls 116 s of the second nanosheets 116. In some embodiments,one of the inner spacers 132 may have a center thickness and an edgethickness less than the center thickness. In addition, the denteddistances DD1, DD2, and DD3 may gradually increase along a directionfrom the substrate 100 to the dummy gate stack 122, namely, DD1<DD2<DD3.However, the embodiments of the present disclosure are not limitedthereto, in other embodiments, the dented distances DD1, DD2, and DD3may be the same, namely, DD1=DD2=DD3. Alternatively, the denteddistances DD1, DD2, and DD3 may be equal to zero, namely, the innerspacers 132 have the outer sidewalls (not shown) aligned with thesidewalls 116 s of the second nanosheets 116. In this case, the innerspacers 132 may be formed as rectangle shape and have the outersidewalls substantially perpendicular to the top surface of thesubstrate 100. That is, the inner spacer material 130 on the sidewalls(FIG. 16) is etched and the inner spacer material 130 filled in thecavities 14 are not etched during the said plasma dry etching.

Referring to FIG. 17 and FIG. 18, a strained material 140 (or a highlydoped low resistance material) are epitaxially grown from the bufferlayer 216. FIG. 18 is the corresponding fragmentary cross-sectional viewof FIG. 8 taken along the line A-A′. In some embodiments, the strainedmaterial 140 is used to strain or stress the second nanosheets (whichmay be referred to as channel members) 116 and the fins 111. Herein, thestrained material 140 may be referred to as S/D regions 140. In thecase, the strained material 140 includes a source disposed at one sideof the dummy gate stack 122 and a drain disposed at another side of thedummy gate stack 122. The source covers an end of the fins 111, and thedrain covers another end of the fins 111. The S/D regions 140 areabutted and connected to the second nanosheets 116, while the S/Dregions 140 are electrically isolated from the first nanosheets 114 bythe inner spacers 132. In some embodiments, as shown in FIG. 18, the S/Dregions 140 extends beyond the top surface 112 t of the nanosheet stacks112. However, the embodiments of the present disclosure are not limitedthereto, in other embodiments, the top surface 140 t of the S/D regions140 is substantially aligned with the top surface 112 t of the nanosheetstacks 112.

In some embodiments, the S/D regions 140 is derived from the material ofthe buffer layer 216. For example, when the buffer layer 216 ispolysilicon layer, the strained material 140 may be a silicon-containingmaterial. In some other embodiments, the S/D regions 140 include anyacceptable material, such as appropriate for p-type FinFETs. Forexample, if the liner layer 134 is silicon, the S/D regions 140 mayinclude SiGe, SiGeB, Ge, GeSn, or the like. In some alternativeembodiments, the S/D regions 140 includes any acceptable material, suchas appropriate for n-type FinFETs. For example, if the liner layer 134is silicon, the S/D regions 140 may include silicon, SiC, SiCP, SiP, orthe like. In some embodiments, the S/D regions 140 are formed by MOCVD,MBE, ALD, or the like.

In some embodiments, the S/D regions 140 may be doped with a conductivedopant. For example, the S/D regions 140, such as SiGe, may beepitaxial-grown with a p-type dopant for straining a p-type FinFET. Thatis, the S/D regions 140 is doped with the p-type dopant to be the sourceand the drain of the p-type FinFET. The p-type dopant includes boron orBF₂, and the S/D regions 140 may be epitaxial-grown by LPCVD processwith in-situ doping. In some alternative embodiments, the S/D regions140, such as SiC, SiP, a combination of SiC/SiP, or SiCP isepitaxial-grown with an n-type dopant for straining an n-type FinFET.That is, the S/D regions 140 is doped with the n-type dopant to be thesource and the drain of the n-type FinFET. The n-type dopant includesarsenic and/or phosphorus, and the S/D regions 140 may beepitaxial-grown by LPCVD process with in-situ doping.

As a result of the epitaxial-grown process used to form the S/D regions140, the cross section of the S/D regions 140 may have a diamond orpentagonal shape as illustrated in FIG. 8. However, the embodiments ofthe present disclosure are not limited thereto. In other embodiments,the cross section of the S/D regions 140 also have a hexagonal shape, apillar shape, or a bar shape. In some embodiments, as shown in FIG. 8,adjacent S/D regions 140 are separated from each other after theepitaxial-grown process is completed. Alternatively, adjacent S/Dregions 140 may be merged.

After forming the inner spacers 132 and the S/D regions 140, a releasingnanosheet process is performed by following steps. For clarity, theforming steps are shown in FIG. 19 to FIG. 24 which correspondfragmentary cross-sectional views of the semiconductor device in FIG. 9to FIG. 13 taken along the line A-A′ and the forming steps are alsoshown in FIG. 26 to FIG. 29 which correspond fragmentary cross-sectionalviews of the semiconductor device in FIG. 10 to FIG. 13 taken along theline B-B′.

Referring to FIG. 9 and FIG. 19, a contact etch stop layer (CESL) 142over the S/D regions 140 and an interlayer dielectric (ILD) layer 144over the CESL 142. In some embodiments, the CESL 142 conformally coversthe S/D regions 140 and the sidewalls of the outer sidewall 128 s of thespacers 128. For clarity, the CESL 142 is not illustrated in perspectiveviews of FIG. 9. The CESL 142 may include silicon nitride, siliconoxynitride, silicon nitride with oxygen (O) or carbon (C) elements,and/or other materials; and may be formed by CVD, PVD (physical vapordeposition), ALD, or other suitable methods.

In addition, in order to illustrate the features behind the frontportion of the ILD layer 144, some front portions of the ILD layer 144are not shown in FIG. 9 and subsequent figures, so that the innerfeatures may be illustrated. It is appreciated that the un-illustratedportions of the ILD layer 144 still exist. In some embodiments, the ILDlayer 144 includes silicon oxide, silicon nitride, silicon oxynitride,phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), spin-onglass (SOG), fluorinated silica glass (FSG), carbon doped silicon oxide(e.g., SiCOH), polyimide, and/or a combination thereof. In some otherembodiments, the ILD layer 144 includes low-k dielectric materials.Examples of low-k dielectric materials include BLACK DIAMOND® (AppliedMaterials of Santa Clara, Calif.), Xerogel, Aerogel, amorphousfluorinated carbon, Parylene, BCB (bis-benzocyclobutenes), Flare, SILK®(Dow Chemical, Midland, Mich.), hydrogen silsesquioxane (HSQ) orfluorinated silicon oxide (SiOF), and/or a combination thereof. Inalternative embodiments, the ILD layer 144 include one or moredielectric materials and/or one or more dielectric layers. In someembodiments, the ILD layer 144 is formed to a suitable thickness byFCVD, HDPCVD, SACVD, spin-on, sputtering, or other suitable methods. Forexample, an interlayer dielectric material layer (not shown) isinitially formed to cover the isolation regions 115, the dummy gatestack 122, and the spacers 128. Subsequently, a thickness of theinterlayer dielectric material layer is reduced until the dummy gatestack 122 is exposed, so as to form the ILD layer 144. The process ofreducing the thickness of the interlayer dielectric material layer maybe achieved by a CMP process, an etching process, or other suitableprocesses. In the case, the top surface 144 t of the ILD layer 144 maybe coplanar with the top surface 122 t of the dummy gate stack 122.

Referring to FIG. 9 and FIG. 20, the dummy gate stack 122 is removed toform a gate trench 16. The ILD layer 144 and the CESL 142 may protectthe S/D regions 140 during removing the dummy gate stack 122. As shownin FIG. 9, the nanosheet stacks 112, the buffer layers 216, and thebottom semiconductor layers 214 are exposed the gate trench 16. In someembodiments, the dummy gate stack 122 may be removed by using plasma dryetching and/or wet etching. When the dummy gate electrode is polysiliconand the ILD layer 144 is silicon oxide, a wet etchant such as a TMAHsolution may be used to selectively remove the dummy gate electrode. Thedummy gate dielectric layer is thereafter removed by using anotherplasma dry etching and/or wet etching.

Referring to FIG. 10, FIG. 21, and FIG. 25, a first etching process isperformed to remove the first nanosheets 114 and a portion of the bottomsemiconductor layers 214. In the case, the first nanosheets 114 may becompletely removed to form a plurality of gaps 18 (which may be referredto as second gaps) between the second nanosheets 116. Accordingly, thesecond nanosheets 116 are separated from each other by the gaps 18. Inother words, as shown in FIG. 25, the second nanosheets 116 aresuspended. The opposite ends of the suspended second nanosheets 116 areconnected to S/D regions 140, as shown in FIG. 10. Herein, the suspendedsecond nanosheets 116 may be referred to as channel members 116. Itshould be noted that the inner spacers 132 may be referred to as abarrier for protecting the S/D regions 140 from damaging during thefirst etching process. Herein, the etching process may be referred to asnanosheet formation or releasing nanosheet process. Although the channelmembers 116 in cross-section illustrated in FIG. 26 are rectangle-likeshape (which has rounded corners, a flat top surface connecting twoadjacent rounded corners, and a flat bottom surface connecting otheradjacent rounded corners), the embodiment of the disclosure is notlimited thereto. In some alternative embodiments, the channel members116 in cross-section may be circular shape, elliptical shape, or thelike.

In some embodiments, a height 18 h of the gaps 18 may be 5 nm to 30 nm.In the present embodiment, the second nanosheets 116 include silicon,and the first nanosheets 114 include silicon germanium. The firstnanosheets 114 may be selectively removed by oxidizing the firstnanosheets 114 using a suitable oxidizer, such as ozone. Thereafter, theoxidized first nanosheets 114 may be selectively removed from the gatetrench 16. In some embodiments, the first etching process includes a dryetching process to selectively remove the first nanosheets 114, forexample, by applying an HCl gas at a temperature of about 20° C. toabout 300° C., or applying a gas mixture of CF₄, SF₆, and CHF₃. Herein,as shown in FIG. 21, vertically stacked nanosheets 116 may be referredto as semiconductor nanosheet stacks or channel stack of the n-typeand/or p-type semiconductor device, alternatively.

On the other hand, as shown in FIG. 21, the bottom semiconductor layers214 may be partially removed in the first etching process to form a gap218 (which may be referred to as first gap) between the buffer layer216, a remaining portion of the bottom semiconductor layers 214 a, andthe fin 111. In the present embodiment, the bottom semiconductor layers214 and the first nanosheets 114 have the same material, such as silicongermanium. In the case, the bottom semiconductor layers 214 exposed bythe gate trench 16 may be selectively removed by oxidizing the bottomsemiconductor layers 214 using a suitable oxidizer, such as ozone.Thereafter, the oxidized bottom semiconductor layers 214 may beselectively removed from the gate trench 16. In some embodiments, thesemiconductor layers 214 and the first nanosheets 114 have the sameetching selectivity in the first etching process. Since the thickness(i.e., T3 in FIG. 1) of the bottom semiconductor layer 214 is thinnerthan the thickness (i.e., T1 in FIG. 1) of the first nanosheets 114, alateral removal amount of the bottom semiconductor layer 214 is greaterthan a lateral removal amount of the first nanosheets 114. Accordingly,the gap 218 may have a width 218 w greater than or equal to a width 18 wof the gaps 18 or a width 16 w of the gate trench 16. In someembodiments, the gap 218 further extends below the S/D regions 140. Thatis, the gap 218 may be in contact with the remaining portion of thebottom semiconductor layers 214 a directly under the S/D regions 140.

Referring to FIG. 11, FIG. 22, and FIG. 26, a bottom dielectric material219 is formed to fill in the gate trench 16, the gaps 18, and the gap218. In some embodiments, the bottom dielectric material 219 includesilicon oxide (SiO), silicon nitride (SiN), aluminum oxide (AlO), or acombination thereof, and may be formed by CVD, ALD or any suitablemethod. As shown in FIG. 22, a portion of the bottom dielectric material219 may fully fill in the gap 218. As shown in FIG. 11, FIG. 22, andFIG. 26, another portion of the bottom dielectric material 219 may wrapthe second nanosheets 116 and conformally cover the gate trench 16 toform a U-shape cross-section. In some embodiments, the bottom dielectricmaterial 219 further covers the top surfaces of the CESL 142 and the ILDlayer 144. For clarity, the bottom dielectric material 219 on thesidewalls of the gate trench 16 and on the top surfaces of the CESL 142and the ILD layer 144 is not illustrated in perspective views of FIG.11.

Referring to FIG. 12, FIG. 23, and FIG. 27, a second etching process isperformed to remove the bottom dielectric material 219 in the gatetrench 16 and the gaps 18. In some embodiments, the bottom dielectricmaterial 219 on the top surfaces of the CESL 142 and the ILD layer 144is also removed, and a portion of the ILD layer 144 may be consumedduring the second etching process. In some embodiments, the secondetching process includes a wet etching process, a dry etching process,or a combination thereof. When the bottom dielectric material 219 isSiO₂ and the ILD layer 144 is SiO₂, a wet etchant such as HF may be usedto selectively remove the bottom dielectric material 219. Since thesecond etching process may etch a material in flat areas and/or greatgap faster than a material in small gap (e.g., slits), a remainingportion of the bottom dielectric material 219 may be left in the gap 218to form a bottom dielectric layer 220, as shown in FIG. 23 and FIG. 27.In the case, the bottom dielectric layer 220 may be sandwiched betweenthe buffer layer 216 and the fin 111. In the present embodiment, thebottom dielectric layer 220 further extend under the S/D regions 140 andmay be in contact with the bottom semiconductor layers 214 a directlyunder the S/D regions 140, as shown in FIG. 23. In some alternativeembodiments, the second etching process includes an anisotropic etchingprocess, an isotropic etching process, or a combination thereof. Takethe anisotropic etching process as example, a sidewall 220 s of thebottom dielectric layer 220 may be aligned with a sidewall 216 s of thebuffer layer 216 as show in FIG. 27. In addition, a sidewall 220 s′(drawn as dotted line) of the bottom dielectric layer 220 may be concavefrom the sidewall 216 s of the buffer layer 216 when the second etchingprocess is an isotropic etching process, as show in the enlarged view ofFIG. 27. After the second etching process, the second nanosheets 116 maybe released again.

Referring to FIG. 13, FIG. 24, and FIG. 28, a gate dielectric layer 152is formed in the gate trench 16 and the gaps 18. FIG. 28 illustrates aclearer view of the gate dielectric layer 152 wrapping the secondnanosheets 116. In addition, the gate dielectric layer 152 conformallycovers the gate trench 16 to form a U-shape cross-section, as shown inFIG. 24. In some embodiments, the gate dielectric layer 152 includes oneor more layers of a dielectric material, such as silicon oxide, siliconnitride, or high-k dielectric material, other suitable dielectricmaterial, and/or combinations thereof. Examples of high-k dielectricmaterial include HfO₂, HfSiO, HfSiON, HfTaO, HfTiO, HfZrO, zirconiumoxide, aluminum oxide, titanium oxide, hafnium dioxide-alumina(HfO₂—Al₂O₃) alloy, other suitable high-k dielectric materials, and/orcombinations thereof. In some embodiments, the gate dielectric layer 152includes an interfacial layer (not shown) formed between the channelmembers and the dielectric material. The gate dielectric layer 152 maybe formed by CVD, ALD or any suitable method. In one embodiment, thegate dielectric layer 152 is formed by using a highly conformaldeposition process, such as ALD in order to ensure the formation of agate dielectric layer having a uniform thickness around each channelmembers. A thickness of the gate dielectric layer 152 is in a range fromabout 0.5 nm to about 3 nm in some embodiments. Thereafter, a gateelectrode 154 is formed on the gate dielectric layer 152 to surroundeach nanosheet or channel member 116. In the case, the gate electrode154 and the gate dielectric layer 152 constitute a gate stack 150, and asemiconductor device 1 of the first embodiment is accomplished, as shownin FIG. 13 and FIG. 24.

The gate electrode 154 may include one or more layers of conductivematerial, such as polysilicon, aluminum, copper, titanium, tantalum,tungsten, cobalt, molybdenum, tantalum nitride, nickel silicide, cobaltsilicide, TiN, WN, TiAl, TiAlN, TaCN, TaC, TaSiN, metal alloys, othersuitable materials, and/or combinations thereof. The gate electrode 154may be formed by CVD, ALD, electro-plating, or other suitable method.The gate dielectric layer 152 and the gate electrode 154 may also bedeposited over the upper surfaces of the ILD layer 144 and the CESL 142.The gate dielectric layer 152 and the gate electrode 154 formed over theILD layer 144 and the CESL 142 are then planarized by using, forexample, CMP, until the top surfaces of the ILD layer 144 and the CESL142 are revealed. In some embodiments, after the planarizationoperation, the gate electrode 154 is recessed and a cap insulating layer(not shown) is formed over the recessed gate electrode 154. The capinsulating layer includes one or more layers of a silicon nitride-basedmaterial, such as SiN. The cap insulating layer may be formed bydepositing an insulating material followed by a planarization operation.

In some alternative embodiments, one or more work function adjustmentlayers (not shown) are interposed between the gate dielectric layer 152and the gate electrode 154. The work function adjustment layers are madeof a conductive material, such as a single layer of TiN, TaN, TaAlC,TiC, TaC, Co, Al, TiAl, HfTi, TiSi, TaSi or TiAlC, or a multilayer oftwo or more of these materials. For the n-type device, one or more ofTaN, TaAlC, TiN, TiC, Co, TiAl, HfTi, TiSi and TaSi is used as the workfunction adjustment layer, and for the p-type device, one or more ofTiAlC, Al, TiAl, TaN, TaAlC, TiN, TiC and Co is used as the workfunction adjustment layer. The work function adjustment layer may beformed by ALD, PVD, CVD, e-beam evaporation, or other suitable process.Further, the work function adjustment layer may be formed separately forthe n−-type device and the p−-type device which may use different metallayers.

Further, although the gate electrode 154 illustrated in FIG. 28 issandwiched between adjacent channel members 116 to separate the gatedielectric layers 152 from each other. However, the embodiment of thedisclosure is not limited thereto, in other embodiments, portions of thegate dielectric layer 154 wrapping the channel members 116 are connectedtogether to form a continuous region. In the case, the gate electrode154 will not be filled into the gaps between the channel members 116.

It should be noted that the bottom dielectric layer 220 below the gatestack 150 is able to block the punch through leakage current betweenadjacent S/D regions 140, thereby enhancing the device performance andthe reliability. In addition, the bottom dielectric layer 220 mayprevent dopants in the substrate 100 (e.g., phosphorous in the n-welland boron in the p-well) from diffusing upwardly into the S/D regions,thereby maintaining the device performance. Furthermore, the saidprocess flow with bottom dielectric layer formation can be easilyintegrated into existing semiconductor fabrication processes. As shownin FIG. 24, the bottom dielectric layer 220 and the bottom semiconductorlayers 214 a are at the same level and connected to each other under theS/D regions 140. Herein, when elements are described as “at the samelevel”, the elements are formed at substantially the same height in thesame layer, or having the same positions embedded by the same layer. Inthe present embodiment, the bottom dielectric layer 220 and the bottomsemiconductor layers 214 a have the same thickness and are bothsandwiched between the fin 111 and the buffer layer 216. From anotherperspective, the bottom dielectric layer 220 and the bottomsemiconductor layers 214 a may be considered as a whole bottom layersandwiched between the substrate 100 and the buffer layer 216.

FIG. 29 to FIG. 30 are cross-sectional views of intermediate stages inthe formation of a semiconductor device in accordance with a secondembodiment of the disclosure.

FIG. 29 illustrates a structure before forming the semiconductor stack102 shown in FIG. 1. After forming the bottom semiconductor material 204and the buffer material 206 on the substrate 100, an implantationprocess 225 may be performed to form a doped region 230 in the buffermaterial 206, the bottom semiconductor material 204, and the substrate100. In some embodiments, the doped region 230 is referred to as ananti-punch-through (APT) region. The conductivity type of the dopantsimplanted in the APT region 230 is the same as that of the wells (notshown) in the substrate 100. On the other hand, the APT region 230 andthe subsequently formed source/drain (S/D) regions 140 (FIG. 30) mayhave different conductivity types. For example, the S/D regions 140 aredoped with an n-type dopant for straining an n-type FinFET, the APTregion 230 is doped with a p-type dopant to reduce the leakage from theS/D regions 140 to substrate 100, and vice versa. In addition, the APTregion 230 may extend under the S/D regions 140 to further reduce theleakage. In some embodiments, the APT region 230 has a dopingconcentration in a range from about 1×10¹¹/cm³ to about 5×10¹³/cm³.

FIG. 29 to FIG. 30 illustrate a method of forming a semiconductor device2 that has detail steps corresponding to FIG. 1 to FIG. 13.Specifically, after forming the APT region 230, the semiconductor stack102 is formed on the buffer material 206, as shown in FIG. 1. Thesemiconductor stack 102, the buffer material 206, the bottomsemiconductor material 204, and the substrate 100 are then patterned toform the plurality of fins 111, as shown in FIG. 2. Thereafter, theplurality of isolation regions 115 are formed aside the hybrid fins 110and the hybrid fins 110 protrude from top surfaces 115 t of theisolation regions 115, as shown in FIGS. 3-4. Next, the dummy gate stack122 is formed across the nanosheet stacks 112, as shown in FIGS. 5-6.The S/D regions 140 are then formed at opposite sides of the dummy gatestack 122, as shown in FIGS. 7-8. Thereafter, the ILD layer 144 areformed on the S/D regions 140, and the dummy gate stack 122 is removedto form the gate trench 16, as shown in FIG. 9. Next, the first etchingprocess is performed to remove the first nanosheets 114 and a portion ofthe bottom semiconductor layer 214, so as to form the first gap 218between the fin 111 and the buffer layer 216 and form the second gaps 18between the second nanosheets 116, as shown in FIG. 10. Next, the bottomdielectric layer 220 is formed in the first gap 218, as shown in FIGS.11-12. The gate stack 150 is then formed in the gate trench 16 and thesecond gaps 18, as shown in FIG. 13. The gate stack 150 may include thegate dielectric layer 152 wrapping the second nanosheets 116 and thegate electrode 154 covering the gate dielectric layer 152. After formingthe gate stack 150, the semiconductor device 2 of the second embodimentis accomplished, as shown in FIG. 13. In the present embodiment, thebottom dielectric layer 220 and the APT region 230 above and below thebottom dielectric layer 220 can further block the punch through leakagecurrent between adjacent S/D regions 140, thereby enhancing the deviceperformance and the reliability.

In some alternative embodiments, the bottom dielectric layer 220 and thegate dielectric layer 152 have the same material. In this case, thesecond etching process as illustrated in FIG. 11 to FIG. 12 may beomitted. That is, the bottom dielectric material (219) may be used asthe gate dielectric layer, the gate electrode 154 then covers on thebottom dielectric material to form the gate stack 150.

According to some embodiments, the semiconductor device includes asubstrate, a plurality of semiconductor nanosheets, a bottom dielectriclayer, and a gate stack. The substrate includes at least one fin. Theplurality of semiconductor nanosheets are stacked on the at least onefin. The bottom dielectric layer is disposed between the at least onefin and the plurality of semiconductor nanosheets. The gate stack wrapsthe plurality of semiconductor nanosheets.

According to some embodiments, a method of forming a semiconductordevice includes: providing an initial structure, wherein the initialstructure comprises a substrate comprising at least one fin, a bottomsemiconductor layer and a nanosheet stack on the at least one fin, andthe semiconductor stack comprises a plurality of first nanosheets and aplurality of second nanosheets stacked alternately; forming a dummy gatestack across the nanosheet stack; forming source/drain (S/D) regions atopposite sides of the dummy gate stack and connected to the nanosheetstack; removing the dummy gate stack; performing a first etching processto remove the plurality of first nanosheets and a portion of the bottomsemiconductor layer, so as to form a first gap between the at least onefin and a bottommost layer of the plurality of second nanosheets and aplurality of second gaps between the plurality of second nanosheets;forming a bottom dielectric layer in the first gap; and forming a gatestack in the second gaps to wrap the plurality of the second nanosheets.

According to some embodiments, a semiconductor device includes asubstrate, a plurality of Si nanosheets, a gate stack, a buffer layer,and a bottom layer. The substrate includes at least one fin. Theplurality of Si nanosheets are stacked on the at least one fin. The gatestack wraps the plurality of Si nanosheets. The buffer layer is disposedbetween the at least one fin and the plurality of Si nanosheets. Thebottom layer is disposed between the at least one fin and the bufferlayer. The bottom layer includes a bottom dielectric layer below thegate stack and a bottom semiconductor layer being aside and in contactwith the bottom dielectric layer.

The foregoing outlines features of several embodiments so that thoseskilled in the art may better understand the aspects of the presentdisclosure. Those skilled in the art should appreciate that they mayreadily use the present disclosure as a basis for designing or modifyingother processes and structures for carrying out the same purposes and/orachieving the same advantages of the embodiments introduced herein.Those skilled in the art should also realize that such equivalentconstructions do not depart from the spirit and scope of the presentdisclosure, and that they may make various changes, substitutions, andalterations herein without departing from the spirit and scope of thepresent disclosure.

What is claimed is:
 1. A semiconductor device, comprising: a substratecomprising at least one fin; a plurality of semiconductor nanosheetsstacked on the at least one fin; a bottom dielectric layer disposedbetween the at least one fin and the plurality of semiconductornanosheets; a buffer layer vertically disposed between the plurality ofsemiconductor nanosheets and the bottom dielectric layer, wherein thebottom dielectric layer has a top surface in physical contact with thebuffer layer; and a gate stack wrapping the plurality of semiconductornanosheets.
 2. The semiconductor device of claim 1, further comprising:source/drain (S/D) regions disposed at opposite sides of the gate stackand connected to the plurality of semiconductor sheets; and a pluralityof inner spacers respectively disposed between the S/D regions and thegate stack.
 3. The semiconductor device of claim 2, wherein the bottomdielectric layer further extends below the S/D regions, and the bottomdielectric layer and the S/D regions are separated by the buffer layer.4. The semiconductor device of claim 2, further comprising a dopedregion disposed in to buffer layer above the bottom dielectric layer andin the substrate below the bottom dielectric layer.
 5. The semiconductordevice of claim 4, wherein the doped region and the S/D regions havedifferent conductivity types.
 6. The semiconductor device of claim 2,further comprising a bottom semiconductor layer disposed between thebuffer layer and the at least one fin, wherein the bottom semiconductorlayer and the bottom dielectric layer are at a same level and connectedto each other.
 7. The semiconductor device of claim 1, wherein athickness of the bottom dielectric layer is less than a distance betweenadjacent two semiconductor nanosheets.
 8. The semiconductor device ofclaim 1, wherein a width of the bottom dielectric layer is greater thanor equal to a width of the gate stack.
 9. A method of forming asemiconductor device, comprising: providing an initial structure,wherein the initial structure comprises a substrate comprising at leastone fin, a bottom semiconductor layer and a nanosheet stack on the atleast one fin, and the nanosheet stack comprises a plurality of firstnanosheets and a plurality of second nanosheets stacked alternately;forming a dummy gate stack across the nanosheet stack; formingsource/drain (S/D) regions at opposite sides of the dummy gate stack andconnected to the nanosheet stack; removing the dummy gate stack;performing a first etching process to remove the plurality of firstnanosheets and a portion of the bottom semiconductor layer, so as toform a first gap between the at least one fin and a bottommost layer ofthe plurality of second nanosheets and a plurality of second gapsbetween the plurality of second nanosheets; forming a bottom dielectriclayer in the first gap, so that the bottom dielectric layer has a bottomsurface in physical contact with the at least one fin; and forming agate stack in the second gaps to wrap the plurality of the secondnanosheets.
 10. The method of claim 9, wherein the bottom semiconductorlayer and the plurality of first nanosheets have a same material. 11.The method of claim 9, wherein the bottom semiconductor layer and theplurality of first nanosheets have a same etching selectivity in thefirst etching process.
 12. The method of claim 9, wherein the formingthe bottom dielectric layer in the first gap comprises: forming a bottomdielectric material to fill in the first and second gaps; and performinga second etching process to remove the bottom dielectric material in thesecond gaps and leave a remaining portion of the bottom dielectricmaterial in the first gap.
 13. The method of claim 9, wherein aremaining portion of the bottom semiconductor layer and the bottomdielectric layer are at a same level and connected to each other. 14.The method of claim 9, wherein a thickness of the bottom semiconductorlayer is less than a thickness of one of the plurality of firstnanosheets.
 15. The method of claim 9, further comprising: forming abuffer layer on the bottom semiconductor layer, wherein the buffer layeris disposed between the bottom semiconductor layer and the nanosheetstack; and after forming the buffer layer, performing an implantationprocess to form a doped region in the buffer layer, the bottomsemiconductor layer, and the substrate.
 16. The method of claim 15,wherein the doped region and the S/D regions have different conductivitytypes.
 17. A semiconductor device, comprising: a substrate comprising atleast one fin; a plurality of Si nanosheets stacked on the at least onefin; a gate stack wrapping the plurality of Si nanosheets; a bufferlayer disposed between the at least one fin and the plurality of Sinanosheets; and a bottom layer disposed between the at least one fin andthe buffer layer, wherein the bottom layer comprises a bottom dielectriclayer below the gate stack and a bottom semiconductor layer being asideand in contact with the bottom dielectric layer.
 18. The semiconductordevice of claim 17, further comprising: source/drain (S/D) regionsdisposed at opposite sides of the gate stack and connected to theplurality of Si nanosheets; and a plurality of inner spacersrespectively disposed between the S/D regions and the gate stack. 19.The semiconductor device of claim 18, wherein the bottom dielectriclayer further extends below the S/D regions, and the bottom dielectriclayer and the S/D regions are separated by the buffer layer.
 20. Thesemiconductor device of claim 17, further comprising a doped regiondisposed in the buffer layer and the at least one fin below the gatestack.